Nickel-metal hydride cells have excellent output characteristics and deliver stable charge and discharge performance. Therefore, nickel-metal hydride cells are now in widespread use for, for example, home electric appliances, mobile devices such as a mobile phone and a notebook personal computer, and rechargeable electric tools.
Nickel-metal hydride cells are also expected as emergency power supplies in reliability-oriented facilities such as a factory and a hospital. Furthermore, nickel-metal hydride cells play a role in alleviating power fluctuations or contribute to peak power reduction, in combination with natural energy-utilizing power generation facilities that generate power varying in amount depending on weather conditions such as wind and sunlight. Therefore, the use of nickel-metal hydride cells is also expected in various fields for the purpose of ensuring power system stability.
Patent Literature 1 discloses the exemplary use of a nickel-metal hydride secondary cell in an interconnected power system. Patent Literature 2 discloses an alkaline secondary cell that includes a positive electrode containing manganese dioxide rather than nickel hydroxide.
With regard to nickel-metal hydride cells, charge-discharge reactions in an alkaline electrolyte may be represented by the following formulae. In the respective formulae, M represents a metal element (hydrogen storage alloy).Positive electrode: Ni(OH)2+OH−NiOOH+H2O+e−  [Formula 1]Negative electrode: M+H2O+e−MH+OH−  [Formula 2]Overall reaction: Ni(OH)2+MNiOOH+MH  [Formula 3]
In charging, nickel hydroxide in a positive electrode is oxidized to nickel oxyhydroxide whereas a metal (hydrogen storage alloy) in a negative electrode turns into a hydride by storing hydrogen generated by the electrolysis of water. In discharging, on the other hand, the metal in the negative electrode releases hydrogen, so that electricity is generated together with water.
Typically, a metal oxide is a poor conductor. In an alkaline secondary cell, nickel hydroxide and manganese dioxide to be used as a positive electrode active material each are a metal oxide with considerably low conductivity. In order to overcome this disadvantage, for example, Patent Literature 3 discloses an active material obtained by adding a higher-order cobalt oxide as a conductive agent to nickel hydroxide. According to the active material, the higher-order cobalt oxide forms a conductive network between the nickel hydroxide particles. This conductive network promotes occurrence of a charge-discharge reaction at the entire nickel hydroxide particles, leading to an increase in capacity.
Cobalt is expensive and has a large specific gravity. Patent Literatures 4, 5 each disclose an inexpensive active material obtained by addition oft as a conductive agent, a graphitized carbon material instead of an expensive higher-order cobalt oxide.
A separator, which constitutes a main part of a secondary cell, plays an important role on cell performance. Specifically, a separator separates a positive electrode from a negative electrode, prevents a short circuit, absorbs and retains an electrolyte, and allows the permeation of gas generated by an electrode reaction. Hence, the separator is required to be hydrophilic.
In regard to this, for example, Patent Literature 6 discloses a technique for providing a cell separator which is suitable for an alkaline secondary cell and has an excellent hydrophilic property under a long-term storage condition or a dried condition, the technique involving sulfonating a polyolefin nonwoven fabric applied with an alkylphosphate anionic surfactant.
Patent Literature 7 discloses a sulfonating process that allows uniform introduction of a sulfonate group and inhibits a decrease in strength, and a method for fabricating a cell separator, the method including moistening a sheet with water and bringing the sheet into contact with sulfuric anhydride gas.